The solder mask is an essential component of modern Printed Circuit Boards, providing critical protection against oxidation, contamination, and electrical shorts during assembly and operational life. Among the various types of solder masks available, Liquid Photoimageable (LPI) Solder Mask has become the industry standard due to its superior resolution, adaptability, and durability.
This document provides a detailed overview of LPI solder mask technology, a comparison with Dry Film Solder Mask, and references to key industry standards including IPC-SM-840 and ISO 9001 requirements for quality assurance.
LPI Solder Mask: Process and Advantages
Liquid Photoimageable (LPI) Solder Mask is a liquid polymer material applied over the PCB surface and selectively hardened by ultraviolet (UV) light exposure. Following development and final thermal curing, it forms a robust protective layer over non-solderable areas.
Manufacturing Process
The typical LPI process flow includes:
- Surface Preparation:
Mechanical scrubbing and chemical cleaning (per IPC-6012 standards) to ensure strong adhesion.
- Application:
Methods include screen printing, curtain coating, or spray coating, depending on PCB complexity and production volume.
- Tack Drying:
Solvent evaporation to stabilize the applied material before exposure.
- UV Exposure:
A phototool or direct imaging system defines solder mask openings with high accuracy.
- Development:
Non-exposed regions are removed using an alkaline solution.
- Final Curing:
Thermal curing to achieve mechanical strength and chemical resistance, as specified in IPC-SM-840 Class T or H qualification tests.
Key Benefits
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High Resolution: Enables fine-pitch components and HDI PCB designs, with minimum feature sizes below 50 microns achievable.
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Durability: Exceptional resistance to solvents, temperature cycling, and mechanical stress.
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RoHS and REACH Compliance: Modern LPI formulations meet global environmental standards, including lead-free soldering temperatures.
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Cost-Effectiveness: Suitable for both prototype and high-volume production without significant material waste.
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Flexibility: Ideal for rigid, flexible, and rigid-flex PCB types.
Dry Film Solder Mask: Overview
Dry Film Solder Mask (DFSM) is a pre-formed sheet of solder mask material laminated onto the PCB under heat and pressure. After lamination, the dry film undergoes similar exposure and development processes as LPI.
Although still in use for specific applications, DFSM is increasingly limited to niche sectors, such as RF/microwave boards and certain rigid-flex assemblies.
Typical dry film photoresist lamination process demonstrating
(a) Lamination of a DFP film on a substrate using the on technique
(b) The substrate after lamination and
(c) After exposure to UV light show the patterns.
LPI vs. Dry Film: Detailed Comparison
Relevant Industry Standards
When implementing solder mask processes, manufacturers are expected to comply with the following industry standards:
1.IPC-SM-840
"Qualification and Performance Specification of Permanent Solder Mask and Flexible Cover Materials"
- Defines performance classes (Class T for telecommunication, Class H for high reliability)
- Establishes solder mask adhesion, chemical resistance, insulation resistance, and thermal cycling requirements.
2.IPC-6012
"Qualification and Performance Specification for Rigid Printed Boards"
- Specifies the quality acceptance criteria for PCB manufacturing, including solder mask coverage and registration.
3.ISO 9001
"Quality Management Systems — Requirements"
- Ensures consistent quality management in solder mask application processes, material selection, and customer satisfaction tracking.
4.RoHS (Directive 2011/65/EU) and REACH (Regulation EC 1907/2006)
- Compliance with environmental directives governing the use of hazardous substances in electronic assemblies.
Conclusion
As PCB designs continue to evolve toward higher complexity, smaller form factors, and greater reliability demands, the choice of solder mask technology becomes increasingly critical. LPI solder mask has emerged as the superior solution, offering excellent resolution, durability, and cost efficiency for both standard and advanced PCB applications.
While Dry Film Solder Mask remains relevant for specific technical cases, particularly in RF and microwave sectors, LPI technology delivers a broader range of benefits aligned with today's high-volume, high-reliability manufacturing requirements.
For PCB projects seeking to meet stringent quality and performance standards such as IPC-SM-840 Class H and ISO 9001 certification, LPI solder mask remains the preferred choice.